This invention relates to the retorting of hydrocarbon-containing solids, particularly shale.
Shale oil is not a naturally occurring product, but is formed by the pyrolysis or distillation of organic material, commonly called kerogen, formed in certain shale-like rock. The organic material has limited solubility in ordinary solvents and therefore cannot economically be recovered by extraction. Upon strong heating, the organic material decomposes into a gas and liquid. Residual carbon typically remains on the retorted shale.
In its basic aspects, the retorting of shale and other similar hydrocarbon-containing solids is a simple operation. The major step involves the heating of the solid material to the proper temperature and the recovery of the vapor evolved. However, for a commercially feasible process, it is necessary to consider and properly choose one of the many possible methods of physically moving the solids through a vessel in which the retorting is to be carried out as well as the many other variances and operating parameters, all of which are interrelated. The choice of a particular method of moving the solids through the vessel must include a consideration of mechanical aspects as well as the chemistry and the processes involved. Further, it is necessary to consider the many possible sources of heat that may be used for the pyrolysis of destructive distillation.
In order to achieve a retorting process that is economically attractive and one which produces the maximum amount of high-quality shale cil, the various operating parameters must be controlled so that the overall process is economical, continuous and highly reliable. Any equipment used in the process must permit a high throughput of material since enormous quantities of shale must be processed for a relatively small recovery of shale oil. Process equipment for shale must have a high thermal efficiency and all mechanical devices should be as simple as possible.
In an effort to provide an economically commercial process, literally hundreds of retorting processes have been proposed, each of which offer a somewhat different choice and/or combination of the many possible operating conditions and apparatus.
One problem with many prior art processes is the quality of the shale oil obtained is relatively low. In many prior art processes long residence times at high temperature result in many secondary and undesirable side reactions, such as cracking, which may increase the production of normally gaseous products and decrease the yield and quality of the normally liquid product.
Another problem with many prior art processes is that a portion of the shale oil is combusted, which also leads to a decrease in the yield of condensable hydrocarbons. Thus, in any process designed to produce the maximum yield of high-quality condensable hydrocarbons, it is preferred that the retorting takes place in the absence of molecular oxygen and that the volatilized hydrocarbons are quickly removed from the retorting vessel in order to minimize deleterious side reactions, such as cracking or polymerization.
The quality and yield of shale oil produced is greatly dependent upon how the retorting process is operated. For example, the raw shale can be heated rapidly or slowly and the shale can be finely divided or vary widely in size. These and other factors greatly influence the quality and quantity of the shale oil produced and the overall thermal efficiency of the process. In essentially all processes for the retorting of shale, the shale is first crushed to reduce the size and time necessary for retorting. Crushing is very expensive and large amounts of energy are required in breaking up the shale and in separating the shale into various size ranges. During the crushing and mining of the shale, it is difficult to obtain uniformly sized pieces and/or to separate the crushed shale into various sizes. Also, it is extremely expensive to crush all of the shale to a very small uniform size. Furthermore, many prior art processes cannot tolerate excessive amounts of shale below about 1/2-inch, while in other prior art processes all of the shale to be processed must be very small, less than 100 mesh, as is required in entrained-bed processes or of relatively uniform small size as is required in fluidized-bed processes. It is, therefore, desirable to have a retorting process which can accommodate intermediate-sized shale thus reducing the amount of crushing required. However, it is further desirable that such a process can handle the fines which necessarily result in any crushing operation.
Another problem with many prior art processes, particularly with countercurrent flow processes, is that after the shale oil has been vaporized, it then comes in contact with countercurrent flowing solids which are at a much cooler temperature, which leads to condensation of a portion of the shale oil and reabsorption of a portion of the vaporized shale oil into the downflowing shale. This condensation and reabsorption leads to coking, cracking and polymerization reactions, all of which are detrimental in regard to producing the maximum yield of condensable hydrocarbons.
In one aspect of the present invention, a countercurrent flowing stripping gas is utilized. Retorting processes using countercurrent flow of stripping gases are well known in the art, for example, U.S. Pat. No. 3,736,247, describes a process wherein shale is fed onto the top of a vertical retort and moves downward countercurrent to the flow of upward flowing stripping gas.
Another aspect of the present invention involves the use of a solid heat-transfer material to provide the necessary heat for the retorting process. The use of solid heat-transfer materials are also known in the art as shown, for example, in U.S. Pat. No. 2,788,314.
The present invention also makes use of the contacting of solids in a spouted bed. Spouted bed contacting of solids is taught in U.S. Pat. Nos. 2,786,280; 3,231,413 and 3,242,586 and also in the book, "Spouted Beds", Mathur et al, Academic Press (1974). Also in a PhD thesis by Berti, "Operational Criterion of a Spouted Bed Oil Shale Retort", Colorado School of Mines (1968), disclosed in a spouted bed shale retorting process wherein the heat for the retorting process is provided by combusting a portion of the shale in the spouted bed. Furthermore, in the Berti process, the shale particles are circulated in the spouted bed until the particle size is reduced by attrition to the point where the retorted shale fines are elutriated out of the top of the bed.